Optical-fiber communication systems may include a transmitter with, for example, a high power, low noise laser source whose optical output is modulated with a wide bandwidth external modulator. Integrated electro-optical modulators are finding increasing use to reduce component physical size while increasing performance. To fabricate an integrated electro-optical modulator, a common substrate material for both the laser and modulator is generally required. Laser sources have been typically fabricated on compound semiconductor materials, to which the optical modulator needs to be adapted.
Electro-optical modulators are based on the principle of modifying the complex optical refractive index in an optical waveguide by the application of an electric field. For this purpose a time-varying electric modulating input signal is transmitted to electrodes located in close proximity to or in contact with an optical waveguide to create an electric field in the optical waveguide. When a constant optical power level is coupled into one end of such a waveguide, the properties of an optical signal output from the other end of the optical waveguide can be modified in response to the modulating input signal applied to the electrodes.
In the case of a dual drive Mach-Zehnder modulator, differential modulating signals are applied to the Mach-Zehnder arms to modify the phase of optical signals propagating through the respective waveguides. When the outputs of the two waveguides are coherently combined in an optical coupling region, the resulting interference produces a change in amplitude of the combined optical signal.
Electro-absorption (EA) modulators typically have a single optical waveguide, in which an electric field applied by the modulating signal modifies optical absorption to produce a change in the amplitude of the optical output signal.
Typically, the optical waveguides of a modulator are fabricated as ridges on a conducting semiconductor substrate. The ridges may comprise a p-n junction with one electrode connection made through the conducting semiconductor substrate, while the other connection is made through an electrode formed on top of the ridge so that the p-n junction can be biased to a desired level and polarity. An electric potential applied between the electrodes controls the electric field strength as well as carrier density within the p-n junction, which in turn produce changes in the refractive index and absorption constant within the optical waveguide.
In a conventional optical modulator, the factors that determine the performance of the modulator are ridge or junction capacitance, electrode capacitance, electrode inductance, length of the modulator and wire-bond pad capacitance. The equivalent circuit model of a conventional modulator is shown in FIG. 1, where Cpad is the pad capacitance, Le is the electrode inductance, Ra is the series resistance of the electrode, Cj is the ridge or junction capacitance, and Rc is the sum of diode forward and contact resistance. The substrate losses are represented by the loss conductance G, corresponding to a loss resistance Rg=1/G. All modulator parameters are functions of the length of the modulator with the exception of the bond pad capacitance. For the circuit diagram shown in FIG. 1, the modulator total capacitance, inductance, resistance and conductance are represented by several of circuit elements. This is usually termed a lumped circuit representation of the modulator. In a distributed circuit model of the modulator, the circuit shown in FIG. 1, is repeated multiple times (excluding Cpad), so that the total capacitance, inductance, resistance and conductance are distributed and represented by multiple circuit elements. In other words, the circuit in FIG. 1 (excluding the bond pad capacitance Cpad at each end of the component chain) can be considered as a unit cell, which is repeated multiple times in order to represent an equivalent circuit of the modulator.
There are three figures of merit which depend on the modulator parameters mentioned above, which will now be briefly explained:
a) The electro-optic (EO) bandwidth;
b) The required radio-frequency (RF) drive level; and
c) The return loss, s11.
The electro-optic (EO) bandwidth increases with lower total shunt capacitance, which is the sum of Cj and Cpad. The ridge capacitance depends on the electrode length, with a longer modulator resulting in lower EO bandwidth. The required radio-frequency (RF) drive level varies inversely with the length of the modulator, with a longer length requiring a lower drive voltage. To achieve lower RF drive voltage, a longer modulator is preferred but the increased total capacitance will decrease the EO bandwidth. Thus a trade-off is needed between bandwidth and the RF drive level requirements. The return loss is very frequency dependent, as it depends on the pad capacitance, electrode inductance, series resistance and total ridge capacitance. In general, higher pad and ridge capacitance result in higher return loss while higher modulator electrode inductance and resistance reduce return loss (assuming modulator ridge has lower impedance than the electrical driving source).
For modulation rates extending to tens of gigabits/second and beyond, the propagation velocity of an optical wave in the optical waveguide is no longer negligible. As the electro-optic interaction length (approximately the length of the modulator signal electrode) becomes comparable to the electrical wavelength at the higher modulation rates, electrical distributed effects become pronounced, so matching of the optical and modulating electrical signal velocities has to be considered. An increase in velocity mismatch tends to reduce the bandwidth of the optical output signal. The electrodes used for applying an electric field to the optical waveguide are designed as electrical transmission lines or micro strip-lines along which the applied electrical modulating signal can propagate as a traveling wave. This is known as a “traveling wave” electrode structure.
In prior art the optical waveguide and the electrical transmission line is typically designed such that the group velocity of the optical wave is matched to the phase velocity of the electrical traveling wave. Velocity matching can be achieved by appropriate optical waveguide dimensioning, micro strip-line dimensioning, addition of reactive electrical components such as inductances and capacitances, or a combination of all three.
The microwave group index and optical group index are important concepts in velocity matching. The more general definitions are:
                    n        =                  c          v                                    [                  Equation          ⁢                                          ⁢          1          ⁢                                          ⁢          a                ]            
where c is the velocity of light in vacuum and v is the optical group velocity or velocity of microwave signal in a medium. More specifically,
                              n          microwave                =                  c                      v            microwave                                              [                  Equation          ⁢                                          ⁢          1          ⁢                                          ⁢          b                ]            
where nmicrowave is the microwave index and vmicrowave is the velocity of an electrical microwave signal traversing the modulator electrodes.
                              n          optical_group                =                  c                      v            optical_group                                              [                  Equation          ⁢                                          ⁢          1          ⁢                                          ⁢          c                ]            
where noptical—group is the optical group index and voptical—group is the group velocity of an optical wave in the waveguide.
For instance, Tanbakuchi (U.S. Pat. No. 7,031,558) discloses an electro-absorption modulator shown as a ridge 50 with a signal electrode 44 in FIG. 2(a). Bond pads 18, 22, are used to connect the EA modulator to a signal source and a termination, respectively. The signal electrode 44 is connected to bond pads 18, 22 by microstrip lines 20 and 24, respectively, which act as inductors, forming matching networks in conjunction with the bond pad capacitances and the bond wires to match the electrical impedance of the modulator ridge 50 to the signal source and a termination, respectively. With reference to FIG. 2(b), the cross-section shows microstrip lines 20 and 24, disposed on an insulating layer 54 using a ground plane provided by electrically conducting layer 55.
The optical and electrical signal velocities are matched, as can be concluded from the phase of scattering parameter S21 of the simulated structure (Tanbakuchi's FIGS. 7C, 6B).
Tanbakuchi's FIG. 7C shows that the electrical phase of an electrical signal propagating from the input to the output of modulator shown in FIG. 6B is linear with frequency. With linear phase, the microwave index, nmicrowave, can easily be estimated by choosing an arbitrary point on FIG. 7C, and applying Equation 2:
                                          β            ⁢                                                  ⁢            L                    =          ϕ                ,                              where            ⁢                                                  ⁢            β                    =                                                                      2                  ⁢                                                                          ⁢                                      π                    ·                    fn                                                  c                            ⁢                                                          ⁢              giving              ⁢                                                          ⁢              n                        =                                          ϕ                ·                c                                            2                ⁢                                                                  ⁢                                  π                  ·                  f                  ·                  L                                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
Substituting L=0.850 mm (from Tanbakuchi's Table 2) and choosing f=48 GHz at which φ=π (or 180°), the value nmicrowave=3.68 can be derived for the microwave index.
The optical group index for group III-V compound semiconductors, in particular InGaAsP, generally lies in the approximate range 3.7-3.9, so the electrical and optical indexes are apparently very well matched.
A variation of the embodiment above is shown in FIG. 2(c), in which the signal electrode is segmented into sections 52 joined in series by microstrip lines 55 on the ridge 50. However, the amount of series inductance, which such microstrip lines can provide, is limited by the space on top of the ridge 50 as well as the impedance per length of the microstrip lines 55.
Skeie (U.S. Pat. No. 5,675,673) proposes a solution that places a series of inductors 531-534 on a separate circuit board 501 as can be seen in FIG. 2(d). While this provides more latitude in choosing inductance values, the separate circuit board requires additional space as well as making more electrical connections necessary to connect to the modulator. Generally this requires more bond wires, whose electrical characteristics, in particular inductance values, are difficult to control, as well as additional bond pads, each of which contribute additional shunt capacitance.
The microwave index of Tanbakuchi-like designs would tend to be in the range of 2.9-3.8, indicating that there is velocity matching between the optical and electrical waves, hence velocity mismatch would not limit the bandwidth in practice. In other words, other factors such as modulator diode capacitance, electrode metal and substrate losses contribute to the predicted bandwidth.
Furthermore, with velocity-matched and similar designs, however, additional filters are often required for increasing the roll-off characteristics of the signal channel bandwidth to avoid interference and cross-talk with neighboring channels. Such additional filters can increase losses, reduce efficiency and contribute to increased size and complexity of the device. It would be advantageous to incorporate the filter function in the integrated optical modulator.
In this disclosure, the above problems are addressed by integrating a network of inductors on the same chip as the modulator, thereby avoiding an increase in device size. Performance of the modulator is further enhanced by departure from velocity-matched conditions in a controlled manner in order to take advantage of the inherent filter characteristics under such conditions.